In addition to causing severe peripheral CD4+ T cell depletion, HIV-1 infects the thymus, causing destruction of both the thymic microenvironment and of thymocytes. Thymic injury and age-related involution may contribute to a failure to achieve adequate immune restoration, even with HAART therapy. Therefore, thymus transplantation is being explored as an approach to improving immune restoration. Under the current funded NCDDG Program, we have shown that porcine thymic grafts in immunodeficient mice support normal development of polyclonal, functional and specifically tolerant human T cells. Importantly, human thymocytes developing in porcine thymus appear to be protected from HlV-1-induced depletion. In this proposal, we will further characterize the level of immunocompetence of human T cells developing in porcine thymus grafts, and will elucidate the mechanism by which porcine thymus confers protection against HIV-1. Specifically, we aim to: 1) Determine the duration and mechanisms of protection of human thymopoiesis from HlV-1-induced destruction, and determine the range of viral strains from which protection will be conferred by porcine thymic grafts. We will evaluate the kinetics of T cell depletion and infection caused by R5, X4 and R5X4 HIV-1 isolates, determine the thymic stromal cell types that are infected with HIV in vivo in HU/HU and SW/HU SCID mice, and compare renewal of thymopoiesis in HIV-infected HU/HU and SW/HU thymic implants following anti-retroviral therapy; 2) Analyze the repertoire and immune function of human T cells developing in pig thymus grafts. We will characterize the TCR diversity and immunocompetence and will evaluate immune responses to protein antigens and to HIV-1. We hypothesize that central tolerance to HIV-1 antigens may prevent HIV reactivity among human thymocytes developing in human but not pig thymic grafts that have been previously infected with HIV-1, and that the restoration of HIV-1-specific immune responses could provide a major advantage to porcine thymic xenografting for the treatment of HIV infection. The use of porcine donors has the additional advantage of providing essentially unlimited amounts of specific pathogen-free, genetically defined xenogeneic thymic tissue.